The present invention relates to band pass filters in general, and more particularly to a narrow band passfilter which is electronically tunable over a predetermined frequency range.
There are already known various filter constructions capable of passing a relatively narrow band of frequencies. Such filters have a high degree of selectivity, which is important for establishing communications with a minimum of interference. This is of special significance in the field of military communications, such as between aircraft or between aircraft and ground, where a plurality of colocated transmitters and receivers operating within the same frequency range such as between 225 and 400 MHz may be operating simultaneously.
The desirability of use of such narrow band pass filters with a high degree of selectivity has already been recognized, and various filters of this type have been developed. So, for instance, there are known tuned cavity filters of the Butterworth and Chebyshev types which have relatively narrow pass bands, such as on the order of 2 percent of the transmission frequency, and relatively low insertion losses such as, for instance, around 1 dB at and around the tuned frequency. Such filters achieve excellent results so long as they are being operated around the tuned frequency. However, tuning of such filters to a given frequency is relatively time-consuming, so that conventionally constructed tuned cavity filters of this type are not suited for use in communication links utilizing frequency-hopping techniques even at relatively low frequency-hopping rates.
To make filters of this type usable in arrangements utilizing frequency hopping, it was previously attempted to electronically tune the filter at any given instant of time to the respective narrow frequency band which includes the transmission frequency band. To this end, it was proposed to use a tuned cavity filter with tapped resonators, wherein a set of tuning capacitors was arranged in the tuned cavity at a tapping region of the respective resonator bar, each of the capacitors being individually switchable by an electronic switching circuit between its active and inactive states so as to contribute or not to contribute to the total capacitance of the respective set and thus to the resonance frequency of the tuning capacitor-resonator bar circuit. The filter of this construction was of the combline type and was designed for use in low power receiver front ends.
Experience with tuned-cavity combline filters of this type using the electronically switchable capacitors arranged within the tuned cavity around the tapping regions of the resonator bars has shown that they have excessive insertion losses and other disadvantageous properties which make this filter hardly usable in high-power transmissions, if at all. The problems encountered in this particular filter construction are at least partially attributable to the accommodation of the tuning capacitors and the associated electric circuitry in the tuned cavity where they are influenced by the electromagnetic field permeating the tuned cavity.